Highly Reproducible and Sensitive Electrochemiluminescence Biosensors for HPV Detection Based on Bovine Serum Albumin Carrier Platforms and Hyperbranched Rolling Circle Amplification

In situ generated CdTe quantum dot-encapsulated hafnium polymer membrane to boost electrochemiluminescence analysis of tumor biomarkers

Exploring the construction of an interface with bright emission, fabulous stability, and good function to develop high-performance electrochemiluminescence (ECL) biosensors for tumor biomarkers is in high demand but faces a huge challenge. Herein, we report an oriented attachment and in situ self-assembling strategy for one-step fabrication of CdTe QD-encapsulated Hf polymer membrane onto an ITO surface (Hf-CP/CdTe QDs/APS/ITO). Hf-CP/CdTe QDs/APS/ITO is fascinating with excellent stability, high ECL emission, and specific adsorption toward ssDNA against dsDNA and mononucleotides (mNs). These interesting properties make it an ideal interface to rationally develop an immobilization-free ECL biosensor for cancer antigen 125 (CA125), used as a proof-of-concept analyte, based on target-aptamer recognition-promoted exonuclease III (Exo III)-assisted digestion. The recognition of ON by CA125 leads to the formation of CA125@ON, which hybridizes with Fc-ssDNA to switch Exo III-assisted digestion, decreasing the amount of Fc groups anchored onto the electrode's surface and blocking electron transfer. As compared to the case where CA125 was absent, significant ECL emission recovery is determined and relies on CA125 concentration. Thus, highly sensitive analysis of CA125 against other biomarkers was achieved with a limit of detection down to 2.57 pg/mL. We envision this work will provide a new path to develop ECL biosensors with excellent properties, which shows great potential for early and accurate diagnosis of cancer.

Isothermal Reciprocal Catalytic DNA Circuit for Sensitive Analysis of Kanamycin

Inappropriate use of veterinary drugs can result in the presence of antibiotic residues in animal-derived foods, which is a threat to human health. A simple yet efficient antibiotic-sensing method is highly desirable. Programmable DNA amplification circuits have supplemented robust toolkits for food contaminants monitoring. However, they currently face limitations in terms of their intricate design and low signal gain. Herein, we have engineered a robust reciprocal catalytic DNA (RCD) circuit for highly efficient bioanalysis. The trigger initiates the cascade hybridization reaction (CHR) to yield plenty of repeated initiators for activating the rolling circle amplification (RCA) circuit. Then the RCA-generated numerous reconstituted triggers can reversely stimulate the CHR circuit. This results in a self-sufficient supply of numerous initiators and triggers for the successive cross-invasion of CHR and RCA amplifiers, thus leading to exponential signal amplification for the highly efficient detection of analytes. With its flexible programmability and modular features, the RCD amplifier can serve as a universal toolbox for the high-performance and accurate sensing of kanamycin in buffer and food samples including milk, honey, and fish, highlighting its enormous promise for low-abundance contaminant analysis in foodstuffs.

Dumbbell hybridization chain reaction coupled with positively charged Au@luminol nanoparticles for enhanced electrochemiluminescent sensing of exosomal miRNA-21

MicroRNAs (miRNAs) are important cancer biomarkers in cancer cell-derived exosomes. Herein, positively charged Au@luminol nanoparticles ((+)Au@luminol NPs) with enhanced electrochemiluminescence (ECL) and extreme stability were firstly established for the sensitive detection of miRNA-21 in exosome. In the presence of miRNA-21, dumbbell hybridization chain reaction (DHCR) happened at gold nanoparticles and ZIF-67 metal-organic framework modified glass carbon electrode (AuNP/ZIF-MOF/GCE) with the help of dumbbell DNA fuel strands (DHP1 and DHP2). The formed DHCR polymers were negatively charged and could electrostatically adsorb numbers of (+)Au@luminol NPs to produce strong ECL signal. Combing DHCR signal amplification with (+)Au@luminol NPs enhancer, sensitive detection of miRNA-21 realized with a detection limit of 0.43 fM. Moreover, the proposed method was successfully applied for the analysis of miRNA-21 in serum samples of healthy individuals and breast cancer patients, indicating a potential application value in early clinical diagnosis.

Advanced technologies towards improved HPV diagnostics

Persistent infection with high-risk types of human papillomaviruses (HPV) is a major cause of cervical cancer, and an important factor in other malignancies, for example, head and neck cancer. Despite recent progress in screening and vaccination, the incidence and mortality are still relatively high, especially in low-income countries. The mortality and financial burden associated with the treatment could be decreased if a simple, rapid, and inexpensive technology for HPV testing becomes available, targeting individuals for further monitoring with increased risk of developing cancer. Commercial HPV tests available in the market are often relatively expensive, time-consuming, and require sophisticated instrumentation, which limits their more widespread utilization. To address these challenges, novel technologies are being implemented also for HPV diagnostics that include for example, isothermal amplification techniques, lateral flow assays, CRISPR-Cas-based systems, as well as microfluidics, paperfluidics and lab-on-a-chip devices, ideal for point-of-care testing in decentralized settings. In this review, we first evaluate current commercial HPV tests, followed by a description of advanced technologies, explanation of their principles, critical evaluation of their strengths and weaknesses, and suggestions for their possible implementation into medical diagnostics.

Highly specific and sensitive sandwich-type electrochemiluminescence biosensor for HPV16 DNA detection based on the base-stacking effect and bovine serum albumin carrier platform

The detection of specific DNA sequences and the identification of single nucleotide polymorphisms are important for disease diagnosis. Herein, by combining the high specificity of the base-stacking effect with the high reproducibility of bovine serum albumin (BSA) modified electrodes and the high loading performance of DNA nanoclews (DNA NCs), a novel sandwich-type electrochemiluminescence (ECL) biosensor is reported for the highly specific detection of HPV16 (chosen as the model target). The capture probes are loaded by BSA carrier platforms modified on the gold electrode surface to improve reproducibility. DNA NCs loaded with a large amount of Ru(phen)32+ worked as signal probes. The template probe is composed of the complementary strand of the target and two free nucleic acid anchors at the head and tail. In the presence of the target DNA, the template probes can form stacked base pairs with target, generating high base-stacking energy. This results in the shorter free anchors of template probes being able to bind to the capture and signal probes. This eventually forms a sandwich structure that allows Ru(phen)32+ to be near the electrode surface, producing an ECL signal. There is a linear relationship between the signal and the target concentration range from 10 fM to 100 pM, with a detection limit of 5.03 fM (S/N=3). Moreover, the base-stacking effect has single base recognition ability for base pairs, effectively avoiding false positive signals. The results of this strategy for clinical samples are consistent with classical methods.

An ultrasensitive fluorescence aptasensor for SARS-CoV-2 antigen based on hyperbranched rolling circle amplification

Sensitive and accurate diagnosis of SARS-CoV-2 infection at early stages can help to attenuate the effects of the COVID-19. Compared to RNA and antibodies detection, direct detection of viral antigens could reflect infectivity more appropriately. However, it is still a great challenge to construct a convenient, accurate and sensitive biosensor with a suitable molecular recognition element for SARS-CoV-2 antigens. Herein, we report a HRCA-based aptasensor for simple, ultrasensitive and quantitative detection of SARS-CoV-2 S1 protein and pseudovirus. The aptamer sequence used here is selected from several published aptamers by enzyme-linked oligonucleotide assay and molecular docking simulation. The sensor forms an antibody-target-aptamer sandwich complex on the surface of microplates and elicits HRCA for fluorescent detection. Without complicated operations or special instruments and reagents, the aptasensor can detect S1 protein with a LOD of 89.7 fg/mL in the linear range of 100 fg/mL to 1 μg/mL. And it can also detect SARS-CoV-2 spike pseudovirus in artificial saliva with a LOD of 51 TU/μL. Therefore, this simple and ultrasensitive aptasensor has the potential to detect SARS-CoV-2 infection at early stages. It may improve the timeliness and accuracy of SARS-CoV-2 diagnosis and demonstrate a strategy to conduct aptasensors for other targets.

Haem-Mediated Albumin Biosensing: Towards Voltammetric Detection of PFOA

The haem group is a promising redox probe for the design of albumin-based voltammetric sensors. Among the endogenous ligands carried by human serum albumin (hSA), haem is characterised by a reversible redox behaviour and its binding kinetics strongly depend on hSA's conformation, which, in turn, depends on the presence of other ligands. In this work, the potential applicability of haem, especially hemin, as a redox probe was first tested in a proof-of-concept study using perfluorooctanoic acid (PFOA) as model analyte. PFOA is known to bind hSA by occupying Sudlow's I site (FA7) which is spatially related to the haem-binding site (FA1). The latter undergoes a conformational change, which is expected to affect hemin's binding kinetics. To verify this hypothesis, hemin:albumin complexes in the presence/absence of PFOA were first screened by UV-Vis spectroscopy. Once the complex formation was verified, haem was further characterised via electrochemical methods to estimate its electron transfer kinetics. The hemin:albumin:PFOA system was studied in solution, with the aim of describing the multiple equilibria at stake and designing an electrochemical assay for PFOA monitoring. This latter could be integrated with protein-based bioremediation approaches for the treatment of per- and polyfluoroalkyl substances polluted waters. Overall, our preliminary results show how hemin can be applied as a redox probe in albumin-based voltammetric sensing strategies.

DNA tetrahedron-besieged primer and DNAzyme-activated programmatic RCA for low-background electrochemical detection of ochratoxin A

Ochratoxin A (OTA) is the most toxic class of ochratoxins and has become a major threat to the environment, humans and animals. Therefore, research on the methods for its detection is also more urgent. Herein, we propose a low-background electrochemical biosensor based on a DNA tetrahedron-besieged primer and a DNAzyme-activated programmatic rolling circle amplification (RCA) that can be ultimately utilized for OTA detection in wine samples. Low-background detection can be achieved using the besieged primer via sequenced assembly of DNA tetrahedral nanostructures so that non-specific extensions of primer can be avoided. The target OTA-mediated DNAzyme activation initiates the programmatic RCA. Additionally, the catalytic property of silver nanoclusters (AgNCs) is integrated with the electrochemical assay to achieve high sensitivity for OTA detection. Benefiting from the aforementioned processes, a low-background, and highly sensitive electrochemical biosensor has been successfully constructed. This design is capable of detecting OTA at concentrations from 1 pg/mL to 10 ng/mL, and its lowest concentration limit is 0.773 pg/mL. Simultaneously, its validation in the detection of actual samples reveals that the proposed electrochemical biosensor has a lot of potential in food safety and environmental detection.

Recent trends and advancements in electrochemiluminescence biosensors for human virus detection

Researchers are constantly looking to find new techniques of virus detection that are sensitive, cost-effective, and accurate. Additionally, they can be used as a point-of-care (POC) tool due to the fact that the populace is growing at a quick tempo, and epidemics are materializing greater often than ever. Electrochemiluminescence-based (ECL) biosensors for the detection of viruses have become one of the most quickly developing sensors in this field. Thus, we here focus on recent trends and developments of these sensors with regard to virus detection. Also, quantitative analysis of various viruses (e.g., Influenza virus, SARS-CoV-2, HIV, HPV, Hepatitis virus, and Zika virus) with a specific interest in Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) was introduced from the perspective of the biomarker and the biological receptor immobilized on the ECL-based sensors, such as nucleic acids-based, immunosensors, and other affinity ECL biosensors.

Biometric Photoelectrochemical-Visual Multimodal Biosensor Based on 3D Hollow HCdS@Au Nanospheres Coupled with Target-Induced Ion Exchange Reaction for Antigen Detection

Three-dimensional (3D) hollow photoactive nanomaterials can enhance light capture due to the light scattering benefiting from the unique hollow nanostructures, which contributes to the decrease in energy loss and the electron-hole recombination during the process of photoelectric conversion. Herein, a 3D hollow HCdS@Au nanosphere synthesized by the templated-assisted method and photodeposition is employed to construct a multimodal sensing platform by combining the photoelectrochemical (PEC) biosensor with colorimetric analysis and photothermal imaging. In the presence of target carcinoembryonic antigen (CEA), a sandwich structure is formed on magnetic beads based on the dual-aptamer recognition, followed by the initiation of rolling circle amplification (RCA) to bind numerous CuO-DNA probes. Upon stimulation by chlorhydric acidic, a large number of Cu²⁺ is released from CuO, which could interact with yellow HCdS@Au on electrode to produce dark CuS by ion exchange. As a result, with increased CEA level, the photocurrent is weakened and the color of electrode interface is changed from yellow to dark, which thus facilitates the PEC and colorimetric detection of CEA. Simultaneously, the formed CuS with highly photothermal effect can achieve qualitative visual analysis of CEA using a portable infrared thermal imager. This work exhibits an excellent performance for sensitive and selective detection of CEA in the dynamic working range from 0.015 to 2.4 ng/mL with a detection limit as low as 3.5 pg/mL. Moreover, the proposed PEC biosensor is successfully applied to CEA determination in human serum, which holds great promise in accurate analysis of biomarkers and early diagnosis of diseases in the clinic.

Strategies for Enhancing the Sensitivity of Electrochemiluminescence Biosensors

Electrochemiluminescence (ECL) has received considerable attention as a powerful analytical technique for the sensitive and accurate detection of biological analytes owing to its high sensitivity and selectivity and wide dynamic range. To satisfy the growing demand for ultrasensitive analysis techniques with high efficiency and accuracy in complex real sample matrices, considerable efforts have been dedicated to developing ECL strategies to improve the sensitivity of bioanalysis. As one of the most effective approaches, diverse signal amplification strategies have been integrated with ECL biosensors to achieve desirable analytical performance. This review summarizes the recent advances in ECL biosensing based on various signal amplification strategies, including DNA-assisted amplification strategies, efficient ECL luminophores, surface-enhanced electrochemiluminescence, and ratiometric strategies. Sensitivity-enhancing strategies and bio-related applications are discussed in detail. Moreover, the future trends and challenges of ECL biosensors are discussed.

Nucleic acid isothermal amplification-based soft nanoarchitectonics as an emerging electrochemical biosensing platform

The emergence of nucleic acid isothermal amplification strategies based on soft nanoarchitectonics offers a new dimension to the traditional electrochemical technique, particularly because of its flexibility, high efficiency, and increased sensitivity for analytical applications. Various DNA/RNA isothermal amplification strategies have been developed for the design and fabrication of new electrochemical biosensors for efficient and important biomolecular detection. Herein, we provide an overview of recent efforts in this research field and the strategies for signal-amplified sensing systems, with their biological applications, current challenges and prospects in this promising new area.

Sensitive Electrochemiluminescence Biosensor Based on the Target Trigger Difference of the Electrostatic Interaction between an ECL Reporter and the Electrode Surface

The discrepancy of the electrostatic interaction of negatively charged signal molecules to long and short DNA strands of the modified electrode surface has been used for the first time to the develop an electrochemiluminescence (ECL) biosensor for human papillomavirus 16 (HPV 16) DNA detection. The short single-stranded capture probe (CP) was modified first on the surface of the gold electrode, which only has a small amount of negative charge. The electrostatic interaction between the negatively charged tris(2,2'-bipyridyl) ruthenium(II) chloride hexahydrate-doped SiO₂ nanoparticles (Ru@SiO₂ NPs) and CP is weak, hence Ru@SiO₂ NPs easily diffuse to the surface of the electrode to generate a strong ECL signal. Hybrid chain reaction (HCR) amplification products (long strand dsDNA) were prepared in homogeneous solution in advance. When the target was present, the dsDNA can be connected on the electrode surface and cause the enhancement of the negative charge on the electrode surface. Owing to electrostatic interaction and steric hindrance, Ru@SiO₂ NPs are difficult to diffuse to the electrode surface, resulting in a significantly reduced ECL signal. The decrease of ECL signal is linearly correlated with the logarithm of the HPV concentration under optimal conditions, with the detection range being 0.1 fM -5 pM with a limit of 1.41 aM. This innovative methodology expands the application of electrostatic interaction in ECL sensing, but can also easily develop biosensors for detecting other targets by changing the DNA sequence used in this strategy.

Interference Reduction Biosensing Strategy for Highly Sensitive microRNA Detection

MicroRNAs are potential biomarkers for human cancers and other diseases due to their roles as post-transcriptional regulators for gene expression. However, the detection of miRNAs by conventional methods such as RT-qPCR, in situ hybridization, northern blot-based platforms, and next-generation sequencing is complicated by short length, low abundance, high sequence homology, and susceptibility to degradation of miRNAs. In this study, we developed a nicking endonuclease-mediated interference reduction rolling circle amplification (NEM-IR-RCA) strategy for the ultrasensitive and highly specific detection of miRNA-21. This method exploits the advantages of the optical properties of long-lived iridium(III) probes, in conjunction with time-resolved emission spectroscopy (TRES) and exponential rolling circle amplification (E-RCA). Under the NEM-IR-RCA-based signal enhancement processes, the limit of detection of miRNA-21 was down to 0.0095 fM with a linear range from 0.05 to 100 fM, which is comparable with the conventional RT-qPCR. Unlike RT-qPCR, the strategy was performed at a lower and constant temperature without heating/cooling cycles and reverse transcription. The strategy could clearly discriminate between matched and mismatched targets, demonstrating high specificity. Moreover, the potential application of this method was demonstrated in cancer cells and mouse serum samples, showing good agreement with RT-qPCR results. Apart from miRNA-21 detection, this platform could be also adapted for detecting other miRNAs, such as let-7a and miRNA-22, indicating its excellent potential for biomedical research and clinical diagnostics.

Self-extending DNA-Mediated Isothermal Amplification System and Its Biosensing Applications

Signal amplification is critical to achieving sensitive biosensing, but complex strategies often bring problems like system instability, false positive, or narrow target spectrum. Here, a self-extending DNA-mediated isothermal amplification (SEIA) system with simple reaction components is introduced to achieve rapid, robust, and significant signal amplification. In SEIA, based on spontaneous refolding of specific DNA domains and using the previous generation product as a template, a DNA strand can extend continuously in an approximate exponential growth pattern, which was accurately predicted by our formula and well supported by AFM results. Based on a set of proof-of-concept experiments, it was proved that the SEIA system can output different signals and flexibly integrate various functional nucleic acids, which makes it suitable for different scenarios and realizes broad-spectrum target detection. Taking into account the advantages of simplicity, flexibility, and efficiency, the SEIA system as an independent signal amplification module will enrich the toolbox of biosensing design.

Recent advances of functional nucleic acids-based electrochemiluminescent sensing

Electroluminescence (ECL) has been used in extensive applications ranging from bioanalysis to clinical diagnosis owing to its simple device requirement, low background, high sensitivity, and wide dynamic range. Nucleic acid is a significant theme in ECL bioanalysis. The inherent versatile selective molecular recognition of nucleic acids and their programmable self-assembly make it desirable for the robust construction of nanostructures. Benefiting from their unique structures and physiochemical properties, ECL biosensing based on nucleic acids has experienced rapid growth. This review focuses on recent applications of nucleic acids in ECL sensing systems, particularly concerning the employment of nucleic acids as molecular recognition elements, signal amplification units, and sensing interface schemes. In the end, an outlook of nucleic acid-based ECL biosensing will be provided for future developments and directions. We envision that nucleic acids, which act as an essential component for both bioanalysis and clinical diagnosis, will provide a new thinking model and driving force for developing next-generation sensing systems.